Process gases, such as those resulting from gas plants and refineries or as a by-product of other industrial processes, may contain an amount of hydrogen sulfide. The recovery of sulfur from these process gases provides a valuable sulfur product. In addition, the sulfur is also preferably removed from the process gases prior to the disposal of the gas, such as through flaring or venting of the process gas to the atmosphere, in order to reduce any atmospheric pollution which may result therefrom.
Accordingly, various sulfur extraction and recovery systems have been developed. For instance, a conventional sulfur recovery process known in the industry as the “Claus process” is often utilized for the recovery of sulfur from hydrogen sulfide containing gases. Specifically, the Claus process utilizes a two step process which results in the production of elemental sulfur and a “tail gas” from the hydrogen sulfide.
The first step in the Claus process is a thermal step which combines the process gas containing the hydrogen sulfide with air or an oxygen containing gas and heating it to the necessary temperature for combustion. The air is used to facilitate the combustion of the hydrogen sulfide. The following reactions both occur in the thermal step:H2S+ 3/2O2→SO2+H2O  (1)2H2S+SO2→3S+2H2O  (2)An amount of both sulfur dioxide and sulfur are typically formed in the thermal step. In addition, amounts of hydrogen sulfide and sulfur dioxide typically remain unreacted.
The products resulting from the thermal step (together with unreacted hydrogen sulfide) then undergo a second catalytic step. The catalytic step combines the products from the thermal step and hydrogen sulfide with a catalyst which results in the production of elemental sulfur and a tail gas which includes a relatively small amount of hydrogen sulfide. In particular, hydrogen sulfide is reacted with sulfur dioxide produced in the thermal step over a catalyst which is utilized to increase the rate of the chemical reaction. Where insufficient hydrogen sulfide remains unreacted from the thermal step to proceed with the catalytic step, a portion of the process gas may bypass the thermal step for use in the catalytic step. The following reaction is the predominant reaction which occurs in the catalytic step:2H2S+SO2→3S+2H2O  (2)
In order to improve the efficiency of the catalytic step, the catalytic step may be repeated to remove additional amounts of sulfur. Typically, the catalytic step is performed in 2 or more, typically about 2 to 4, catalytic reaction stages.
Further, where desired, the catalytic step may be followed by one or more further processing steps such as further processing of the tail gas utilizing a tail gas clean-up unit (“TGCU”) or performing a SUPERCLAUS™ step utilizing a SUPERCLAUS™ catalytic reaction apparatus. The SUPERCLAUS™ catalytic reaction apparatus utilizes a specialized or improved catalyst to achieve higher conversion rates to sulfur. In particular, a selective oxidation catalyst is utilized which oxidizes the hydrogen sulfide at a high efficiency. Typically, the SUPERCLAUS™ step is performed as an adjunct to the conventional Claus process and is performed following the catalytic step, or final catalytic reaction stage, of the Claus process.
In order to facilitate or enhance the sulfur recovery rate of the Claus process and the SUPERCLAUS™ step, if utilized, the amount of air or oxygen containing gas combined with the hydrogen sulfide containing process gas in performing the thermal step is preferably optimized. Specifically, an optimal ratio of oxygen to hydrogen sulfide in the process gas, or an optimal hydrogen sulfide content, is preferably maintained. However, depending upon the source of the process gas, both the flow rate and the composition of the process gas tend to vary. A sudden change in the composition or flow rate of the process gas, causing a sudden change in the oxygen demands, is referred to as an “upset condition.” During upset conditions, a resulting alteration in the ratio from the optimal tends to adversely impact upon the sulfur recovery rate. It has been found that even small temporary deviations from the optimal may cause a considerable decrease in the overall performance or efficiency of the sulfur recovery processes.
Conventionally, the air flow to the thermal step in the Claus process is controlled through feed forward and feed back control systems. The feed forward control system provides at least one flow meter for monitoring the flow rate of the hydrogen sulfide containing process gas to the thermal step. Accordingly, an upset related to the flow rate of the hydrogen sulfide containing process gas is detected by the flow meter, allowing the necessary adjustment to the air flow to be made.
In addition, a single sample feed back control system is provided which samples the process gas at a single point or location in the process. Conventionally, the feed back control involves sampling the tail gas which exits the catalytic step at a single location with an air demand analyzer, also referred to as a tail gas analyzer. The amount of air which is combined with the hydrogen sulfide containing process gas prior to the thermal step is then controlled having regard to the relative proportions or content of hydrogen sulfide (H2S) and sulfur dioxide (SO2), and possibly the content of carbonyl sulfide (COS) and carbon disulfide (CS2), which are found in the tail gas.
Basing control of the Claus process on the composition of the tail gas in this manner provides a relatively accurate measure of process conditions. However, the disadvantage of basing control on the composition of the tail gas is that there is a relatively significant time delay between the time that the hydrogen sulfide containing process gas enters the thermal step and the time that the tail gas exits the catalytic step. Depending upon the flow rate of the gases, the number of catalytic reaction stages and other process parameters, a two to three minute time delay or more may be experienced.
As a result, during “upset conditions” where the composition, and thus the air demands, of the hydrogen sulfide containing process gas entering the thermal step changes suddenly, there is a delay of at least about two to three minutes before the control system based upon the composition of the tail gas recognizes the occurrence of the upset conditions. Furthermore, if the amount of air which is combined with the hydrogen sulfide containing process gas is then adjusted based upon upset conditions which occurred two to three minutes earlier, the adjustment may take place at a time when the upset conditions are no longer present. The end result of utilizing the composition of the tail gas as the basis of controlling the amount of air which is added to the hydrogen sulfide containing process gas before the thermal step is that the occurrence of upset conditions continue to reduce the overall efficiency of the Claus process and SUPERCLAUS™ step.
Other single sample feed back control systems suggest the sampling of the process gas nearer the end of the thermal step, as the process gas exits the reaction furnace. However, although sampling the process gas nearer the thermal step increases the speed at which the feed back is provided to the control system, it has been found that such samples provide for a relatively less accurate measure of process conditions as compared with sampling of the tail gas.
Examples of single sample feed back control systems are provided in Canadian Patent Application No. 2,035,554 published Aug. 22, 1991 by Lell, U.S. Pat. No. 3,026,184 issued Sep. 26, 1957 to Karasek and U.S. Pat. No. 6,287,535 issued Sep. 11, 2001 to Schendel. However, for the reasons discussed above, none of these control systems have been found to be fully satisfactory.
Therefore, there is a need in the industry for an improved control process and control system for controlling a sulfur recovery process and system respectively, and preferably for use with the Claus process and the SUPERCLAUS™ step. In particular, there is a need for the control process and system to enhance the overall efficiency or performance of the Claus process, and preferably the SUPERCLAUS™ step, preferably by providing an improved feed back control process and system which are both relatively accurate and relatively fast as compared with conventional feed back control processes and systems used in the industry.